People react as an explosion goes off near the finish line of the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions went off at the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/The Boston Globe, David L Ryan) MANDATORY CREDIT
Gallery: More photos from Boston explosion
Apr. 16, 2013 @ 01:16 PM
Investigators believe the two bombs that ripped through the crowds at the Boston Marathon, killing three people and wounding more than 170, were fashioned out of pressure cookers and packed with shards of metal, nails and ball bearings.